<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <meta http-equiv="X-UA-Compatible" content="ie=edge">
    <title>Document</title>
</head>

<body>
    <div class="app">
        <input type="text" v-model="n1">
        <select v-model="opt">
            <option value="+">+</option>
            <option value="-">-</option>
            <option value="*">*</option>
            <option value="/">/</option>
        </select>
        <input type="text" v-model="n2">
        <input type="button" value="=" @click='calc'>
        <input type="text" v-model='ads'>
    </div>

    <script src="./node_modules/_vue@2.5.22@vue/dist/vue.js"></script>
    <script>
        var vm = new Vue({
            el: '.app',
            data: {
                n1: 0,
                n2: 0,
                ads: 0,
                opt: '+'
            },
            methods: {
                calc() {
                    switch (this.opt) {
                        case '+':
                            this.ads = this.n1 + this.n2
                            break;
                        case '-':
                            this.ads = this.n1 - this.n2
                            break;
                        case '*':
                            this.ads = this.n1 * this.n2
                            break;
                        case '/':
                            this.ads = this.n1 / this.n2
                            break;
                    }
                    // var codeStr = 'parseInt(this.n1)'+this.opt+'parseInt(this.n2)'
                    // this.ads = eval(codeStr)
                }
            }
        })
    </script>
</body>

</html>